As a commonly used image forming apparatus, such as a printer, a facsimile machine, a copier, a plotter, and a multifunction machine combining the functions of two or more of these apparatuses, there is an image forming apparatus which uses a liquid ejection device including a recording head corresponding to a liquid ejection head that ejects droplets of a liquid, such as a recording liquid, for example, and which forms an image by causing the liquid to adhere to a recording medium conveyed past the recording head.
The liquid ejection device including the liquid ejection head includes a serial-type liquid ejection device and a line-type liquid ejection device. The serial-type liquid ejection device performs recording by mounting the liquid ejection head on a carriage and moving the carriage in a main scanning direction perpendicular to a recording sheet feeding direction. The line-type liquid ejection device uses a line-type head in which a plurality of nozzles serving as ejection ports for ejecting liquid droplets are disposed in rows over substantially the entire width of the sheet.
Further, the liquid ejection head is roughly divided into a few types of systems, depending on the type of actuator used for ejecting liquid droplets, such as ink droplets. For example, a piezo system and a bubble jet (registered trademark) system are commonly known. According to the piezo system, one wall of a liquid pressurizing chamber is formed of a relatively thin diaphragm, and a piezoelectric element serving as an electromechanical transducer element is provided for the diaphragm. Application of an electric current causes the piezoelectric element to deform, thereby deforming the diaphragm, changing the pressure in the liquid pressurizing chamber, and ejecting the ink droplets. According to the bubble jet system, a heating element is disposed in a liquid chamber and applied with current to generate bubbles by heating. With the pressure of the bubbles, the ink droplets are ejected.
According to another system using electrostatic force, a diaphragm forming one wall of the liquid chamber and individual electrodes disposed outside the liquid chamber facing the diaphragm are provided, and an electric field is applied between the diaphragm and the electrodes to generate an electrostatic force that deforms the diaphragm, thus changing the pressure and volume in the liquid chamber and ejecting the ink droplets from the nozzles. Hereinafter, devices which generate pressure in the above-described liquid pressurizing chamber or liquid chamber will be collectively referred to as the “device which generates pressure in a liquid pressurizing chamber based on a drive waveform.”
The liquid ejection head ejects liquid droplets from the ejection ports to perform recording. Thus, if the liquid droplets are not ejected for a relatively long time, a solvent of the ink remaining in the ejection ports evaporates and viscosity of the ink is increased. Consequently, the ejection state may become unstable and cause a failure to eject the liquid droplets properly, with a concomitant deterioration in print quality. To prevent such a situation, therefore, a preliminary ejecting operation is performed that discharges the high-viscosity ink by ejecting from the nozzles liquid droplets that do not contribute to the image formation.
The liquid ejection device performing the preliminary ejecting operation includes, for example, a liquid ejection device in which, in successive liquid ejections based on a plurality of drive pulses, the high-viscosity ink is discharged with the liquid ejection speed set at maximum in the first preliminary ejection droplet and thereafter sequentially and gradually reduced to cause preliminary ejection droplets to fly without merging with one another. The liquid ejection speed is further reduced in the last preliminary ejection droplet to minimize the generation of a minute satellite liquid droplet and thereby reduce ink mist. Other known configurations includes devices in which the drive frequency of the liquid ejection head is increased in accordance with the reduction in viscosity of the ink, to thereby reduce the viscosity of the ink in the liquid ejection head to a normal value, or devices in which, to remove the high-viscosity ink, the drive waveform for the preliminary ejecting operation is varied between a preceding preliminary ejecting operation and a subsequent preliminary ejecting operation.
The drive pulse applied to the liquid ejection head in the preliminary ejecting operation is higher than the drive pulse applied to the liquid ejection head in normal image formation. This is because it is naturally desired to apply a relatively high drive pulse to the liquid ejection head to eject the high-viscosity ink. If a relatively high drive pulse is applied to the liquid ejection head from the beginning, however, an excessive load may be placed on meniscus, depending on the viscosity of the ink, and may cause a phenomenon such as nozzle-down (i.e., failure to eject the liquid droplets from the nozzles) and liquid stagnation. Yet none of the conventional configuration described above takes the problem of the load on the meniscus into account or provides a satisfactory solution thereto.
In terms of load on the meniscus, a background liquid ejection device that is disclosed in JP-2010-094871-A is intended to perform the operation of setting the liquid ejection speed to the highest value in the first one of the plurality of drive pulses and thereafter sequentially reducing the liquid ejection speed, i.e., intended to reduce the mist. As is obvious therefrom, this background liquid ejection device is not intended to reduce the excessive load on the meniscus due to the preliminary ejecting operation.
Another background liquid ejection device disclosed in JP-07-290720-A performs the preliminary ejection (alternatively referred to as preparatory ejection) while changing the drive frequency of the liquid ejection head. This background liquid ejection device is intended to efficiently perform the preparatory ejection of the viscosity-increased liquid in a relatively short time by performing the preparatory ejection while increasing the value of the drive frequency of the liquid ejection head. Therefore, the background liquid ejection device disclosed in JP-07-290720-A is neither intended to reduce the excessive load on the meniscus. Even if the control method of this background liquid ejection device is employed to reduce the excessive load on the meniscus, it is complicated and difficult to perform the control while changing the drive frequency.
Yet another background liquid ejection device disclosed in JP-2004-034471-A changes the drive waveform between before and after a group of preliminary ejections. In this case, the time interval of each group of preliminary ejections is of millisecond order. Thus, it is hardly considered that the high-viscosity ink is effectively removed. Further, this background liquid ejection device is not intended to reduce the excessive load on the meniscus.